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Full text of "The manufacture and properties of iron and steel"

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36                                              INTEODUCTIOJST.
mate strength and a much lower elastic limit. The elongation will decrease as the length increases, so that if a length of 15 feet gives a stretch of 15 per cent, a length of 35 feet will not give over 13 per cent.
WELDING.
In the days of wrought-iron, welding was the basis of all forging and of very much structural work. To-day all structural members are of steel, as well as a great proportion of the stock in the shop of the village blacksmith. This soft steel will weld, and the average blacksmith and machinist, to say nothing of some engineers who ought to know better, believe that a welded piece of steel is practically as good as a new bar. As a matter of fact, while a weld is better than nothing, and while it may have half the strength of the natural bar, and may have its full strength, it does not have its toughness and is unfit to use where failure will be dangerous, and where it can be avoided. It is also true that a weld of wrought-iron is entirely unreliable.
STEEL CASTINGS.
A steel casting is a mass of steel poured directly into finished shape from fluid steel made in the regular way. In this country acid open-hearth furnaces are generally used, but in Germany the basic furnace is often employed. Sometimes the Bessemer converter is used for this work. One of the latest forms is known as the Tropenas process. Instead of having the tuyeres in the bottom of the converter, the air is blown at a low pressure upon the surface of the bath. At a point from .four to seven inches above this set of tuyeres is another set, which supplies air to burn the carbonic oxide coming from the metal. This upper row of tuyeres is not operated until the blowing is well under way. The lower tuyeres oxidize the carbon to carbonic oxide (CO), just as in an ordinary converter, while the upper tuyeres burn this to carbonic acid (C02). In this way there is a great increase in the amount of heat produced and the steel will be hotter than if blown in the usual way.
In the steel foundry, it is the practice to put "sink-heads" on steel castings. These are masses of metal that rise above the rest of the casting and are of such size that they stay liquid while the main body is solidifying, and the metal flows from these heads down